Authenticate user via libpam.

libpam-authenticate.c

#include <assert.h>
#include <security/pam_appl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h> // getpass

// apt install libpam0g-dev
// compile with flag -lpam

char *pam_get_str(const pam_handle_t *pamh, int item_type) {
  char *str;
  assert(pam_get_item(pamh, item_type, (const void **)&str) == PAM_SUCCESS);
  return str;
}

int conv(int num_msg, const struct pam_message **msg,
         struct pam_response **resp, void *appdata_ptr) {
  // [...] *resp is a struct pam_response array [...]
  *resp = calloc(num_msg, sizeof(struct pam_response));
  assert(*resp != NULL);
  for (int i = 0; i < num_msg; i++) {
    // getpass returns a statically allocated string
    (*resp)[i].resp = strdup(getpass(msg[i]->msg));
  }
  return PAM_SUCCESS;
}

int main(int argc, char **argv) {
  char *service_name;
  if (argc > 1) {
    service_name = argv[1];
  } else {
    service_name = "test";
  }
  char *user = NULL;
  const struct pam_conv pam_conversation = {conv, NULL};
  pam_handle_t *pamh; // blind structure, see security/_pam_types.h
  int status = pam_start(service_name, user, &pam_conversation, &pamh);
  if (status != PAM_SUCCESS) {
    fprintf(stderr, "pam_start: %s\n", pam_strerror(pamh, PAM_SYSTEM_ERR));
    return 1;
  } else {
    printf("PAM service name \"%s\"\n", pam_get_str(pamh, PAM_SERVICE));
    int status = pam_authenticate(pamh, 0);
    if (status != PAM_SUCCESS) {
      fprintf(stderr, "pam_authenticate: %s\n", pam_strerror(pamh, status));
    } else {
      user = pam_get_str(pamh, PAM_USER);
      fprintf(stderr, "Successfully authenticated user \"%s\".\n", user);
      fprintf(stdout, "%s\n", user);
    }
    assert(pam_end(pamh, status) == PAM_SUCCESS);
    return status == PAM_SUCCESS ? 0 : 2;
  }
}